A gate valve has a body with a flow passage extending through it. A gate cavity intersects the flow passage, and seat rings are located at the upstream and downstream points of intersection. An actuator, which may be mechanical, hydraulic or electrical, moves a gate within the cavity between open and closed positions. In the closed position, a seal surface on the gate contacts the downstream seat ring. The upstream pressure exerts a force pushing the seal surface into tight sealing engagement. The gate has an aperture through it next to the seal surface. When in the open position, the aperture registers with the flow passage.
When the operator begins to open the gate valve, the seal surface of the gate slides across the seat ring as the gate moves to the open position. The upstream pressure force continues to act on the gate until the aperture reaches the seat rings. If the upstream pressure force is high, a considerable friction force results even if the seal surface and the seat rings are smooth surfaces and lubricants are employed. This frictional force can damage the seal surfaces of the seat ring and gate.